The invention relates to imaging devices and more particularly to camera tubes and electrophotographic plates.
Blocking contacts are known in the prior art for reducing the dark currents normally associated with various photoconductor bodies employed in imaging devices. However, to date, no effective high resistance type photoconductive body has been developed for imaging devices having polycrystalline selenium, even where attempts have been made to also incorporate a blocking contact.
In contrast to selenium rectifiers where low resistance hexagonal (also technically known as "trigonal" selenium) polycrystalline selenium layers are conventionally employed primarily as a consequence of desired high forward current, camera tubes and electrophotographic plates have been, in general, fabricated having vitreous (i.e. amorphous) high resistance selenium layers. Such devices, are for example described in:
U.s. pat. No. 3,350,595 issued to W. M. Kramer on Oct. 31, 1967, PA1 U.s. pat. No. 3,405,298 issued to J. Dressner on Oct. 8, 1968, PA1 U.s. pat. No. 3,861,913 issued to C. Chiou on Jan. 21, 1975; and PA1 Great Britain Pat. No. 1,343,197 issued to Tanaka et al and filed on June 14, 1971.
The electrical characteristics of vitreous selenium and hexagonal selenium vary considerably as discussed in an article entitled "Some Investigations on the Electrical Properties of Hexagonal Selenium" by L. M. Nijland, in Philips Res. Rep. 9 (1954), pp. 259-294. Vitreous selenium at 20.degree. C. is highly resistive, where the resistivity may be between 10.sup.10 ohm-cm. and 10.sup.14 ohm-cm., whereas hexagonal selenium is quite conducting and may have a resistivity from a few hundred ohm-cm. to 10.sup.9 ohm-cm. depending upon the concentration of halogens, or thallium impurities, respectively.
Since imaging devices ordinarily must have low dark currents, such devices have generally employed vitreous selenium because it is highly resistive and is able to form a blocking contact with n type semiconductor materials such as cadmium selenide or low work function oxidized metals such as oxidized aluminum. Considerable difficulty has been encountered in fabricating useful camera tube targets employing vitreous selenium as a consequence of the thermal instability of the material which results from its crystallization, even when abutting stabilizing layers are provided, or when arsenic or phosphorus is incorporated into the selenium material to retard crystallization. On the other hand, stable and effective amorphous selenium layers are known for electrophotographic plates, where lag characteristics of the photoconductor are less critical, which incorporate arsenic and tellurium and which do not display this undesired crystallization.
Multilayered electrophotographic plates are known in the prior art which incorporate both the vitreous (high resistance) and polycrystalline (low resistance) forms of selenium. One structure is described in an article entitled "Advances in Xerography: 1958-1962" by C. J. Claus in Photographic Science and Engineering, Vol. 7, No. 1, Jan.-Feb. 1963. The structure described includes a tin oxide coating upon which two selenium overlayers are successively disposed. The first overlayer comprises a panchromatic "control layer" of crystalline selenium and the second comprises a charge storage layer of amorphous selenium.
In general, there is desired a low dark current, thermally stable, high resistance photoconductive body suitable for imaging devices having a simplified construction.